NKT cells with an invariant T cell receptor alpha chain (iNKT cells) are a unique T cell sublineage capable of rapidly producing an array of cytokines when activated through their TCR. In mouse model systems, they have been reported to exert a pivotal role in determining the outcome of a variety of immune responses and pathologic conditions, including the development of autoimmunity, the maintenance of self-tolerance, the response to tumors and infectious agents, and even atherosclerosis development. Moreover, cancer immune therapy clinical trials based on activating iNKT cells have been carried out. iNKT cells are autoreactive for antigens presented by CD1d and their specificity is highly conserved when mouse and human are compared. Until recently, however, it was not known what glycolipids presented by CD1d were the natural antigens that activate iNKT cells, and their reactivity for microbial antigens was controversial. Our preliminary data establish that the majority of iNKT cells are reactive to several categories of bacterial glycolipids that have a terminal, alpha-linked hexose sugar, and that this reactivity is likely to be responsible for protective immune responses. We propose to apply an array of biochemical and immunological techniques, ranging from in vitro TCR binding studies to measurements of immune responses in vivo, in order to understand how bacterial glycolipids are recognized, and how the response to them mediates host protection. Therefore, these studies will provide important information on the basic biology of iNKT cells and their physiologic raison d'etre, including their ability to recognize antigens, the forces driving their selection and conservation, and their participation in host protection, and they may lead to improved vaccines against bacterial pathogens.